Why are there synchronous and asynchronous builds?

[rhashimoto]

WebAssembly currently doesn't have native support for coroutines or continuations. That makes it difficult to connect SQLite with browser resources that are inherently asynchronous, like IndexedDB.

Asyncify is a clever workaround for that restriction. It postprocesses the .wasm file and adds some library support to effectively suspend some WebAssembly calls, run some asynchronous Javascript, and resume executing the WebAssembly. That can be used to bridge C's customarily blocking i/o, which SQLite uses, and the browser's asynchronous APIs (though not the only way).

Asyncify is amazing and awesome, but it isn't free. It makes the WebAssembly substantially larger and slower, particularly with SQLite which is something of a worst case. Developers can decide whether the costs are acceptable for their application, but the asynchronous build should only be used if it is actually needed.